1. Field of the Invention
The present invention relates in general to a flow control valve, and more particularly to a flow control valve for controlling flow rate of a pressurized working fluid fed to an actuator of a power steering device of a motor vehicle.
2. Description of the Prior Art
A power steering device is widely used in a motor vehicle for assisting the driver to smoothly operate the steering wheel. The power steering device is usually equipped with a pump for producing a pressurized working fluid for operating the same. The pump is usually driven by the engine of the vehicle so that flow rate of the pressurized working fluid discharged from the pump varies in proportion to rotational speed of the engine. Accordingly, such a pump is required to supply a sufficient flow Accordingly, such a pump is required to supply a sufficient flow rate of the pressurized working fluid even when the engine speed is low so as to fully assist a driver to smoothly operate the steering wheel. However, when the engine speed is high, flow rate of the pressurized working fluid fed to an actuator of the power steering device must be lowered so as to stiffen the steering response for ensuring the driving safety in a high speed range of the car. Thus, various types of flow control valve have been proposed to control flow rate of the pressurized working fluid.
Referring to FIG. 2, a conventional flow control valve unit will be described in the following.
The flow control valve unit 10 is formed in a housing 12 of a hydraulic pump (not shown). The flow control valve unit 10 comprises a generally cylindrical spool chamber 14 and a spool valve 16 which is axially slidably disposed therein. The spool chamber 14 has a closed end 18, and is communicated through an orifice 20 with a discharge port 22 which is communicated with an actuator (not shown) of a power steering device (not shown). The spool chamber 14 is further communicated with a drain or recirculating port 24 and an inlet port 26, which are disposed on the opposite sides of an outer cylindrical surface of the spool chamber 14, as illustrated. Both the drain port 24 and the inlet port 26 are arranged perpendicular to a longitudinal axis of the spool chamber 14. It should be noted that the inlet port 26 is positioned closer to the orifice 20 than the drain port 24 is, with respect to a direction along the longitudinal axis of the spool chamber 14. The drain port 24 serves to recirculate excessive working fluid to the hydraulic pump so as to control flow rate of the pressurized working fluid flowing to the discharge port 22. The inlet port 26 serves to feed the pressurized working fluid from the hydraulic pump into the spool chamber 14.
The spool valve 16 has first and second land portions 16a 16b and a spool portion 16c which is interposed therebetween. The spool chamber 14 is divided into a first chamber portion 14a which is defined between the orifice 20 and the spool valve 16, and a second chamber portion 14b which is defined between the spool valve 16 and the closed end 18 of the spool chamber 14. A return spring 28 is disposed in the second chamber portion 14b so as to urge the spool valve 16 toward the orifice 20. The discharge port 22 is communicated with the second chamber portion 14b through a communication passage 30.
For the purpose which will be clarified hereinafter, there is provided a notch 32 which is opposed to and aligned with the drain port 24, as illustrated. To construct the drain port 24 and the notch 32, for example, the housing 12 is drilled through and perpendicularly to the spool chamber 14. However, if desired, other methods may be taken to construct the drain port 24 and the notch 32.
Operation of the spool valve unit 10 will be described in the following.
While the pump speed is maintained within a predetermined low range due to a low engine speed, fluid discharge rate of the pump is set to be low. This makes the pressure differential between the first and second chamber portions 14a and 14b small relative to the biasing force of the return spring 28. Therefore, the spool valve 16 takes a first position in which the first land portion 16a is positioned closer to the orifice 20 than the notch 32 and the drain port 24 are, with respect to a direction along the longitudinal axis of the spool chamber 14. That is, the drain port 24 is fully closed by the first land portion 16a so as to prevent the flow of working fluid into the drain port 24. Accordingly, all the working fluid supplied to the first chamber portion 14a is fed to the actuator of the power steering device through the orifice 20 and the discharge port 22. Thus, during a practical low engine speed range, the driver is fully assisted to smoothly operate the steering wheel.
When the pump speed exceeds the predetermined low range, fluid discharge rate of the pump also increases. This makes the pressure differential between the first and second chamber portions 14a and 14b large so as to overcome biasing force of the return spring 28. Therefore, the spool valve 16 is moved toward the closed end 18 of the spool chamber 14. Thus, as is seen from FIG. 2, the spool valve 16 takes a second position in which the drain port 24 is partially opened. Under this condition, as is shown by two arrows of FIG. 2, a part of the pressurized working fluid supplied from the pump flows from the first chamber portion 14a into the drain port 24 through first and second gaps 34 and 36. The first gap 34 is defined between an edge of the drain port 24 and the first land portion 16a of the spool valve 16. The second gap 36 is defined between the first land portion 16 and an edge of the notch 32. The first and second gaps 34 and 36 have the same area defined in a direction along the longitudinal axis of the spool chamber 14, because the first land portion 16a is arranged perpendicular to the cylindrical surface of the spool chamber 14. Due to the flow of working fluid from the first chamber portion 14a into the drain port 24, flow rate of the working fluid into the discharge port 22 decreases so as to stiffen the steering response.
The above-mentioned notch 32 is provided to reduce a side force and a so-called flow noise, which are caused by the flow of working fluid through the narrowly opened first gap 34. In other words, if the notch 32 were not provided, the side force and the flow noise would become relatively large, particularly when the first gap 34 is relatively narrow. Thus, it should be noted that the notch 32 is provided to induce the flow of working fluid through the second gap 36 into the drain port 24, thereby reducing the flow rate of working fluid flowing through the first gap 34 and reducing the side force and the flow noise caused thereby. The side force is exerted on the spool valve 16 in a direction substantially perpendicular to the longitudinal axis of the spool chamber 14, which direction is an upward direction in FIG. 2. Therefore, the side force causes uneven abrasion of the spool valve 16 and the spool chamber 14. The flow noise occurs due to, for example, agitation and cavitation of the working fluid in the drain port 24.
However, the effect of the provision of the notch 32 is still unsatisfactory to reduce the side force and the flow noise which are caused by the flow of working fluid through the narrowly opened first gap 34.